Magnetic separator having a plurality of inclined magnetic separation boxes

ABSTRACT

A separator device for separating magnetic particles from a fluid current containing magnetic and non-magnetic particles in which separation of the magnetic particles is effected by passing the fluid through a separation device in the form of a number of parallel plates to which the particles will adhere under the action of a strong magnetic field applied to the separator device, the plates of the separator device being inclined at a small angle to the horizontal, the separator being cyclically operated to cause zones of strong and substantially zero magnetic field whereby separation of the magnetic particles from the remainder of the fluid current is effected.

United States Patent 119 Jones [11] 1 3,822,016 1451 Jul 2, '1974PLURALITY OF INCLINED MAGNETIC SEPARATION BOXES [76] Inventor: GeorgeHenry Jones, Connor Downs, Hayle, Cornwall, England [22] Filed: Apr. 17,1972 [21] Appl. No.: 244,786

[52] US. Cl .1 209/222, 209/223 R, 209/232 [51] Int. Cl. 1303c U04 1[58] Field of Search 209/223, 222, 221, 220, 13 9 12243 225,, 3212192 1210/222, 223

[56] References Cited UNITED STATES PATENTS 832,825 10/1906 Wait 209/2221,024,045 4/1912 Weatherby 209/224 1,462,111 7/1923 Jobke 209/2221,601,658 9/1926 Ullrich 209/223 R 1,683,780 9/1928 Mulsmeyer 210/2232,760,638 8/1956 Annettwj. 210/222 2,771,995 11/1956 Noel 3,221,88212/1965 Frantz 3,326,374 7/1963 Jones 209/232 X 3,346,116 10/1967 Jones....210/222 3,693,792 9/1972 Lang 209/219 FOREIGN PATENTS ORAPPLICATIONS 105,831 9/1898 Germany ..209/222 252,034 5/1926 GreatBritain ..209/222 768,451 2/1957 Great Britain ..209/223R PrimaryExaminer-Robert I-lalper 5 7 ABSTRACT A separator device for separatingmagnetic particles from a fluid current containing magnetic and nonmagnetic particles in which separation of the magnetic particles iseffected by passing the fluid through a separation device in the form ofa number of parallel plates to which the particles will adhere under theaction of a strong magnetic field applied to the separator device, theplates of the separator device being inclined at a small angle to thehorizontal, the separator being cyclically operated to cause zones ofstrong and substantially zero magnetic field whereby separation of themagnetic particles from the remainder of the fluid current is effected.

8 Claims, 15 Drawing Figures MAGNETIC SEPARATOR HAVING A PLURALITY OFINCLINED MAGNETIC SEPARATION BOXES FIELD OF THE INVENTION The presentinvention relates to a means for separating solid magnetic particlesfrom a fluid in which they are suspended. I

PRIOR ART I s In known separators, such as are described in theSpecifications of my British Pat. No. 768,451 and my US. Pat. No.3,326,374 a fluid current, carrying magnetic particles of a solid suchas iron, is passed vertically through one or more plate-boxes.

In both cases the method of magnetic separation comprises at least twoprincipal steps in each cycle, namely the passing of particle carryingfluid through a zone of strong magnetic field which include magnetizableplate-like members so that the magnetic particles in the fluid adherethereto, and thereafter passing a flow of scouring fluidthrough saidsame zone after the magnetic field has been reduced to zero. Anadditional step of washing the magnetic particles whilst they areadhering to the plate-like members may be provided between the feed andscour periods. The magnetic field which is applied to the zone in whichthe plate-like members are situated is applied transversely to the flowof the particle carrying fluid. In the former patent the plate-likemembers are stationary and the value of the magnetic field variedbetween maximum and zero. In the latter patent, the plate-boxes aremounted in a circle and rotatable about a central axis so as to passinto and out of zones of magnetic field which are not themselves varied.

If it is required to separate very small magnetic particles or particleswhich are only slightly magnetic then difficulty is encountered since,in the present state of technology, there is a limit to the strength ofmagnetic field economically attainable.

SUMMARY OF THE INVENTION According to the present invention there isprovided means for magnetically separating solid magnetic particles froma fluid current including a magneticparticle separation device havingparallel plates arranged therein and spaced apart to enable a fluidcurrent to flow therethrough, said separation device being in clined atan angle to the horizontal to assist the flow of fluid therethrough;means for applying a magnetic field transversely to the flow of fluidthrough the inclined separation device so as to cause the magneticparticles to adhere to the sides of the plates; means for applying tothe upper end of the separation device in cyclic rotation the fluid feedcurrent from which the magnetic particles are to be removed and a flowof wash fluid to remove the magnetic particles after the magnetic fieldhas been reduced to zero.

BRIEF DESCRIPTION OF DRAWINGS The present invention will now bedescribed in greater detail by way of examples with reference to theaccompanying drawings, wherein:

FIGS. la to 1c. are side elevation, end elevation and plan viewrespectively of a plate-box for use in a separator; e I

FIG. 2a is a side elevation of a first embodiment of separator;

FIG. 2b is a section of part of the separator shown in FIG. 2a;

FIG. 3 is a schematic diagram, in front elevation, of a secondembodiment of separator;

FIG. 4 is a longitudinal section of a separation device which can beused in any of the separators herein described;

FIG. 5 is a'longitudinal section of a modified form of plate-box for usein a separator;

FIG. 6 is a section through a third embodiment of separator;

FIG. 7 is a section through a fourth embodiment of separator;

FIG. 8 is a schematic plan view of a rotary separator such as that shownin FIG. 6 or FIG. 7;

FIG. 9a is an end elevation of an alternative form of plate-box for usein a separator; FIG. 9b is a schematic diagram, in front elevation, of afifth embodiment of separator;

FIG. 10a is a section through part of a sixth embodiment of separator;and

FIG. 10b is a schematic diagram, in front elevation, of the sixthembodiment of separator part of which is shown in FIG. 10a.

DESCRIPTION OF PREFERRED EMBODIMENTS surface of each of the plates 3,run longitudinally of the plate-box 1, from one open end to the other.Twelve bolts 5 are provided for holding the casing 2 together, thecasing 2 being made up of four rectangular plates 6. Two locating rods 7are provided for maintaining the plates 3 in position. The non-groovedmajor surface of the plates 3 are uppermost and each is in contact withthe apices of the grooved surface of the next upper adjacent plate. Theplates 3 are made of a readily magnetizable material. The top and bottomof the casing 2 is made of magnetic material, whilst the sides are madeof a non-magnetic material such as brass.

Referring to FIGS. 2a and 2b a separator 8 for magnetically separatingsolid magnetic particles from a fluid has two electromagnetseachconsisting of a coil 9 anda pole-piece 10. A non-magnetic centralrectangular section 11 separates the pole-pieces 10, one of which isarranged vertically above the other. In the gap separating thepole-pieces l0 and within the rectangular section 11 there is arranged aplate-box separation device 12 which incorporates the plate-box 1. Ascan be more clearly seen in FIG. 2b the pole-pieces 10 are shaped sothat their mutually registering faces are inclinedto the horizontal, soas to be parallel to the flow of fluid in the plate-box 1. Theseparation device 12 is located between these inclined faces of thepole-pieces 10, the device 12 sloping from the input end downwardly tothe output end, a variable-position funnel 13 being provided at theoutput end. A half-butterfly valve 14 is also provided at the output endto minimize the velocitydifl'erences between the layers of fluid flowingThree cam-operated valve stems 16 (only one of which is shown) areprovided at'the input end for controlling inlet valves to the plate-box1.

Referring to FIG. 3 there is shown a second embodiment in which twoseparators 8 are mounted side by side. There are four coils 9 arrangedon the arms of U- shaped magnetic pieces 17 (only the upper of which isshown, the lower being hidden by other parts of the separator). Thereceptacle is in the form of an inclined longitudinal trough which leadsto storage tanks (not shown). A fluid feed arrangement consisting of asupply pipe 18, an impeller 19 and a circulating pipe 20, is connectedup to each of the separators 8. A wash water tank 21 is connected via anassociated nonretum valve 22 to the separators 8. A scour-supply pipe 23is similarly connected via a scour valve 24. The camoperated valve stems16 (FIG. 2) are driven by means of a shaft (not shown) which is coupledby means of a chain to a pulley 25 which is driven by means of a drivegear assembly 26. The funnels 13 are variably positioned in synchronismwith the valve stems 16 by means of a shaft 27.

Referring to FIG. 4 a separation device 12 is shown having a modifiedform of plate-box 28 in which the plates 3 are held by means of apart-wedge member 29 which is held in the plate-box 28 by means of ascrewthreaded member 30. The screw-threaded member 30 is engaged in aflange 31 of the casing of the device 12.

Rotation of the member 30 causes it to push the member 29 into theplate-box 28 so jamming the plates 3 in position, or causes it towithdraw the member 29 from the plate-box so that plates can be removedfor cleaning, repair or any other reason. A guide member 32 is securedto the casing of the device 12 and serves to guide the outflowing fluidsfrom the plate-box 28 into the funnel 13.

At the inlet end of the device 12 each valve stem 16 operates a valve33. The valve 33 shown controls entry of water from a chamber 34. Awater-supply pipe 35 communicates with the chamber 34 to supply water tothe chamber. The valve 33 is lined with a layer 36 of a resilientplastics material or rubber. Alternatively, it

may be lined with a metal. A fluid feed supply pipe 37 is shown adjacentthe chamber 34 but cut off from it.

Referring to FIG. 5 there is shown a second form of plate-box 38 inwhich two dividing plates 39 are provided. A chute member 40 is formedintegral with the casing 41 of the plate-box 38. The plate-box 38 isotherwise similar to the plate-box 1. The dividing plates 39 extend fromthe assembly of plates 3 in the plate-box 38 and so form three supplytroughs 42 for three layers of plates 3. When fluid is supplied from apipe 43 it flows into the uppermost of the three supply troughs 42. Theintermediate supply trough 42 is only supplied with fluid when theuppermost supply trough overflows. Similarly the lowermost supply trough42 is only supplied with fluid when the intermediate supply trough 42overflows. The dividing plates 39 therefore substantially equalize thepressure head of fluid flowing through the plates 3. This eliminates thenecessity for the half-butterfly valve 14 when the plate-box 38 is usedin place of the plate-box l.

Referring to FIGS. 1 to 5, in operation the separation process iscarried out in three successive stages. Firstly the cam operation opensa first valve 33 which allows fluid feed which is being continuouslycirculated through the pipe 20 in order to prevent settling of solids inthe pipe, into the plate-box for a certain length of time. During thistime the coils 9 are energized by a DC. current so that magneticparticles in the fluid flowing through the plate-box are attracted tothe plates 3 due to the pole-pieces l0 magnetizing the plate-box. As canbe seen from FIG. lb the apices of the triangular grooves 4 areuppermost so that the magnetic flux is concentrated at the edges of thegrooves 4 where one plate 3 contacts the next lower adjacent plate 3.Thus most of the magnetic particles are held at those points ofconcentrated flux.

Secondly, after the flow of fluid feed has been cut off due to the firstvalve 33 closing, a second valve 33 opens and allows wash water to enterthe plate-box so that non-magnetic contaminants are washed away. Duringthis time the coils 9 are kept energized and the supply of wash water isof low pressure.

Thirdly, after the second valve 33 is closed, a third valve 33 is openedallowing scouring water to be passed through the plate-box. Thehalf-butterfly valve 14 is turned so as not to impede flow. The coils 9are deenergized or they can be energized by a current flowing in adirection reverse to the previous direction of energization current soas to counteract the remanent field on de-magnetization as disclosed inmy British Patent 963,193. Thus the magnetic particles are no longermagnetically held in the plate-box and are carried out of the plate-boxin a current of scouring water.

The output from the separator during the three stages is guided by thefunnels 13 into three respective compartments of the receptacles 15.

This three-stage cycle is carried out repetitively giving a cyclicseparator.

Referring to FIG. 6 there is shown a rotary separator in whichseparation devices 12 are carried on arms 44 of a spider-rotor 45 of anon-magnetic material and which is rotatably driven by a verticallyarranged shaft 46. There are two pairs of magnetizing stations 47 (onlyone pair of which is shown) arranged so that there are magnetizingstations 47 at intervals around the rotor 45. Each pair of magnetizingstations 47 is formed by a pair of U-shaped magnetic pieces 17 which arejuxtaposed one on top of the other, in spaced relationship to define airgaps in which separation devices 12 can be accommodated. Support members48 are provided for supporting the upper magnetic piece 17. The air gapsformed by the pieces 17 slope outwardly downwards from the rotor so thatoutput from the rotary separator is received at points around the outerperimeter of the separator. A drive motor 49 is coupled by means of agearbox 50, mounted on the upper U-shaped magnetic piece, to the shaft46 which runs in a lower bearing 51 supported on the lower magneticpiece 17.

Referring to FIG. 7 which also relates to a rotary separator, instead ofa non-magnetic spider-rotor 45, a solid discoidal magnetic rotor 52 isprovided so eliminating the need for a lower magnetic piece 17. As canbe seen from FIG. 7, the discoidal rotor is horizontally arranged andsupported on a base member housing the gearbox 50. The rotor has acentral flat circular area surrounded by a downwardly inclined area onwhich the separator boxes 12 are located therearound. The drive motor 49and gearbox 50 located with a supporting base plate are now arrangedunder the separator. The rotor 52 serves to complete the magneticcircuit of between magnetizing stations 47.

In operation the separation devices on the rotor can either be placed inthe magnetizing stations singly or in groups. The separation process iscarried out, as previously described, in three stages. The feeding offluid and washing away of non-magnetic contaminants is done in thosedevices 12 during magnetization of the the magnetizing stations 47 whilethe scouring operation is carried out in those devices 12 during little,non or reverse magnetization of the magnetizing stations 47. The rotorcan either rotate in one direction continuously or in steps. Also therotor can oscillate from one position to another and back again to thefirst position. The rotor 53 can be either a non-magnetic spider or amagnetic solid discoidal rotor.

Referring to FIG. 9a another form of plate-box is shown. The maindifference between this plate-box and that shown in FIG. lb resides inthe construction of the casing 2 and, in particular, in the plate 6which forms the top plate of the casing 2, i.e., that plate on which theupper magnetic piece 17 bears. This top plate is formed as a pistonmember 106 having at one end a radially extending peripheral flange 107which serves to limit the inward stroke of the piston member into theplate-box. The piston member 106 is dowelled to plates 6' forming thesides of the casing 2 by means of dowels 108. Between the edges of theplates 6 and the flange 107 there is provided a soft tubular packing 109which serves to seal the plate-box. The weight of the upper magneticpiece 17 augmented by the force of magnetic attraction serves to pushthe piston member 106 into the plate-box so as to compress the plates 3together.

Referring to FIG. 9b there is shown an embodiment in which the plate-boxshown in FIG. 90 could be used. In this embodiment the upper magneticpiece 17 is mounted on rods 110 supported by an arch-support lll.Lifting jacks 112 are fixed on top of the arch support 111 for acting onthe rods 110, which pass through apertures (not shown) in the support111, to raise the upper magnetic piece 17 which in the normaloperational position rests on the plateboxes. Thus the plateboxes can beremoved for cleaning or repair whenever it is desired to do so.

Referring to FIGS. 10a and 10b there is shown a sixth embodiment of amagnetic separator, in which the plate-box shownin FIG. 9a could beused. In this embodiment the pole-pieces of the magnetic pieces 17present pole faces inclined to the horizontal. The platebox is insertedbetween the inclined pole faces. To pre-' vent the upper magnetic piece17 sliding on the inclined surface presented by the plate-box and socausing damage to the rods 110 and other parts of the separator, guidebars 113 are provided.

The inclination of the separation devices 12 in all the examples ofseparator described is between 0 and 90 being preferably in the range 5to 45. A small inclination results in a low rate of flow of the fluidand water through the plate-boxes giving a high proportion of the 6magnetic particles separated from the fluid. A larger inclination gives,a fast flow with a decrease in the proportion of magnetic particlesextracted.

Although the plates 3 in the plate-boxes described had their non-groovedmajor surfaces uppermost, they can clearly be arranged with theirgrooved major surfaces uppermost in the plate-boxes. Also various shapesof groove, other than triangular can be used.

The plates 3 may be of iron, steel, magnetic stainless steel or cobaltiron alloy (Permendur etc.) and may be unprotected against wear andcorrosion or protected by chromium plating, chromium diffusion or othersuit able method. The plates 3 are preferably made of low carbon steelor stainless steel containing approximately 13 percent chromium withlittle or no nickel, one variety of which is known as stainless: iron.

The grooved plates 3 may be replaced by other members of magneticmaterial, e.g. steel wool, expanded metal, wire gauge, steel balls,steel rods of circular or other suitable section, or twisted bars ofsquare, triangular or other suitable section.

It should be appreciated that when grooved plates 3 are used they tendto move within the plate-boxes because of large changes in magneticfield due to switching on and off and reversal of the field.

Clearly although the operation of the separators has been described asbeing a three-stage process, the washing operation could be omitted togive a two-stage operation.

In all the separators the feeding and washing take place in a magneticfield whilst the scouring can take place in a very reduced or zeromagnetic field strength, or in a reversed magnetic field. The flow ofscour water may be at a pressure between 10 and 60 pounds per squareinch.

Instead of the magnetic air gaps being inclined using the shapedpole-pieces 10, the entire magnetic fieldproducing structure can betilted.

Instead of the screw-threaded member 30, a spring may be provided tokeep the part-wedge member 29 in place in the plate-box 28.

The switches controlling the energization of the coils 9 may becontrolled by the same cam arrangement as controls the valve stems 16.

The operation of the device could be hydraulically, solenoid orcompressed air controlled instead of being controlled by a camarrangement.

In a preferred arrangement a number of cams on a rotating shaft controlelectrical switches directly and may control some valves directly andother valves indirectly by means of solenoids or air valves or acombination of both.

Clearly in the rotary separators permanent magnets could be used inplace of the electromagnets made up of the magnetic pieces 17 and coils9.

The above described magnetic separators will remove particles ofmetallic iron, biotite, tourmaline, iron oxides, nickel-iferouspyrrhotite or serpentines, ilmenite, compounds intermediate betweenrutile and ilmenite, iron oxide, manganese oxides, iron carbonates, ironsilicates, garnets, epidote, chromite, Wolfram or compounds containingiron from mixtures containing particles of quartz, felspar, bauxite,clay, phosphates, apatite, magnesite, strontium, or barium sulphides,scheelite, or carbonates or other non-magnetic substances.

What I claim and desire to secure by Letters Patent is: v

1. A rotary magnetic separator for separating solid magnetic particlesfrom a fluid current, comprising a rotatable horizontally arrangednon-magnetic spider rotor having a plurality of inclined arms slopingoutwardly downwardly from the center of the rotor; a plurality ofmagnetic separation boxes arranged on the arms of the spider rotor; aseries of flat plates located in each magnetic separation box, saidplates having triangular grooves on the lower major surface thereof, theapices of said grooves contacting the upper major surface of the nextlower plate; upper and lower U-shaped magnetic pieces; means forsupporting the upper magnetic pieces in relation to the lower magneticpieces to form a pair of air-gaps therebetween and between which saidseparation boxes pass as the rotor rotates, said air gaps being inclinedoutwardly downwards to correspond to the inclination of the separationboxes; energizing coils located on the ends of the U-shaped member forproviding when energized a magnetic field across the air-gap so as tostrongly magnetize the plates of a separation box located therein; andmeans for rotating said spider rotor in cyclic stepped manner foralternately moving each separation box into a zone of strong magneticfield within the air-gap for fluid current to be fed thereinto andthereafter into a zone of zero magnetic field, located midway betweenthe pair of air-gaps defined by said U-shaped magnetic pieces, for theapplication of scouring fluid to remove the magnetic particles leftadhering to the plates of the separation box.

2. A rotary magnetic separator according to claim 1, wherein said spiderrotor is carried on a vertically arranged central shaft which runs in abearing located on the lower U-shaped magnetic piece and wherein a drivemotor and gearbox are mounted on the upper U- shaped member to drivesaid shaft which extends therethrough.

3. A rotary magnetic separator for separating solid magnetic particlesfrom a fluid current including a rotatable horizontally arranged soliddiscoidal magnetic rotor having a central flat circular area, surroundedby a downwardly inclined annular area; a plurality of magneticseparation boxes carried about said annular area of the discoidal rotor;a series of flat plates located in each magnetic separation box, saidplates having triangular grooves on the lower major surface thereof, theapices of said grooves contacting the upper major surface of the nextlower plate; a U-shaped magnetic piece having outwardly downwardlyinclined ends; means for supporting the U-shaped piece such that theinclined ends thereof are located over diametrically opposite separationboxes thereby forming an air-gap between the top of the separation boxesand the outwardly downwardly inclined ends; energizing coils located atend portions of said U-shaped member for providing a magnetic fieldacross the airgap to strongly magnetize the plates of a separation boxlocated therein; and means for rotating said discoidal rotor in cyclicstepped manner for each separation box in turn to first be moved into azone of strong magnetic field within the air-gap and secondly into azone of substantially zero magnetic field whereby the fluid currentcontaining magnetic particles to be separated therefrom is first fedinto the separation box in a zone of strong magnetic field whereafterscouring fluid is fed into the separation box in a zone of substantiallyzero magnetic field so as to remove the magnetic particles left adheringto the plates of the separation box.

4. A rotary magnetic separator according to claim 3, wherein there aresixteen magnetic separation boxes arranged in equi-spaced relationaround the annular area of said discoidal rotor, and wherein twoadditional zones of strong magnetic field are provided to formadditional air-gaps at relationship to said first pair of air-gaps, saidfour air-gaps each being of such a width that two magnetic separationboxes can be accommodated therein simultaneously whilst a pair ofseparation boxes on each side thereof is in a zone of zero magneticfield.

5. A magnetic separator for separating solid magnetic particles from afluid current including a pair of magnetic separation boxes; a series offlat plates located in each magnetic separation box, said plates havingtriangular grooves on the lower major surface thereof, the apices ofsaid grooves contacting the upper major surface of the next lower plate;upper and lower U-shaped magnetic pieces; means for supporting the uppermagnetic piece in relation to the lower magnetic piece to form a pair ofair-gaps therebetween in which the magnetic separation boxes are locatedin inclined relation to the horizontal; energizing coils carried at endportions of the U-shaped members for providing a magnetic field acrossthe air-gap when said coils are energized thereby to strongly magnetizethe plates of the separation box for removing magnetic particles from afluid current, during a fluid current feed cycle, said removed magneticparticles adhering to said plates; a main supply of electric current forsaid coils for strong magnetization of said plates; means for passing asmall electric current reverse to that of said main supply through saidcoils for reducing the remanent magnetic field to substantially zerowhen said main supply is interrupted during a scouring period to therebyremove the magnetic particles adhering to the plates of the separationbox; and jacking means for raising the upper U- shaped magnetic piece inrelation to the lower magnetic piece to facilitate removal of theseparation boxes from said air-gaps.

6. A magnetic separator according to claim 5, wherein said upperU-shaped magnetic piece is carried by a pair of rods, and wherein anarch-support carries a pair of lifting jacks to which the ends ofrespective rods are attached.

7. A. magnetic separator according to claim 6, wherein the top-plate ofthe separator box is formed as a piston member having a radiallyextending peripheral flange, the face of the upper U-shaped magneticpiece bearing on the piston member, a packing member being providedbetween the walls of the separator box and the piston member, and guidebars being provided to cooperate with the upper U-shaped magnetic pieceto prevent it sliding on the inclined surface of the separator boxes.

8. A magnetic separator according to claim 5, wherein each separator boxincludes two dividing plates located between the triangular groovedplates so as to divide said plates into three groups within saidseparator box, said dividing plates being longer than the triangulargrooved plates and arranged to extend out of the inlet side of theseparator box in an upwardly inclined direction owing to the inclinationof the separator box, said dividing plates thus forming together with achute extending from the base of the separator box, three supply troughsfor the three groups of triangular grooved plates.

1. A rotary magnetic separator for separating solid magnetic particles from a fluid current, comprising a rotatable horizontally arranged non-magnetic spider rotor having a plurality of inclined arms sloping outwardly downwardly from the center of the rotor; a plurality of magentic separation boxes arranged on the arms of the spider rotor; a series of flat plates located in each magnetic separation box, said plates having triangular grooves on the lower major surface thereof, the apices of said grooves contacting the upper major surface of the next lower plate; upper and lower U-shaped magnetic pieces; means for supporting the upper magnetic pieces in relation to the lower magnetic pieces to form a pair of air-gaps therebetween and between which said separation boxes pass as the rotor rotates, said air gaps being inclined outwardly downwards to correspond to the inclination of the separation boxes; energizing coils located on the ends of the U-shaped member for providing when energized a magnetic field across the air-gap so as to strongly magnetize the plates of a separation box located therein; and means for rotating said spider rotor in cyclic stepped manner for alternately moving each separation box into a zone of strong magnetic field within the air-gap for fluid current to be fed thereinto and thereafter into a zone of zero magnetic field, located midway between the pair of air-gaps defined by said Ushaped magnetic pieces, for the application of scouring fluid to remove the magnetic particles left adhering to the plates of the separation box.
 2. A rotary magnetic sepparator according to claim 1 wherein said spider rotor is carried on a vertically arranged central shaft which runs in a bearing located on the lower U-shaped magnetic piece and wherein a drive motor and gearbox are mounted on the upper U-shaped member to drive said shaft which extends therethrough.
 3. A rotary magnetic separator for separating solid magnetic particles from a fluid current including a rotatable horizontally arranged solid discoidal magnetic rotor having a central flat circular area, surrounded by a downwardly inclined annular area; a plurality of magnetic separation boxes carried about said annular area of the discoidal rotor; a series of flat plates located in each magnetic separation box, said plates having triangular grooves on the Lower major surface thereof, the apices of said grooves contacting the upper major surface of the next lower plate; a U-shaped magnetic piece having outwardly downwardly inclined ends; means for supporting the U-shaped piece such that the inclined ends thereof are located over diametrically opposite separation boxes thereby forming an air-gap between the top of the separation boxes and the outwardly downwardly inclined ends; energizing coils located at end portions of said U-shaped member for providing a magnetic field across the air-gap to strongly magnetize the plates of a separation box located therein; and means for rotating said discoidal rotor in cyclic stepped manner for each separation box in turn to first be moved into a zone of strong magnetic field within the air-gap and secondly into a zone of substantially zero magnetic field whereby the fluid current containing magnetic particles to be separated therefrom is first fed into the separation box in a zone of strong magnetic field whereafter scouring fluid is fed into the separation box in a zone of substantially zero magnetic field so as to remove the magnetic particles left adhering to the plates of the separation box.
 4. A rotary magnetic separator according to claim 3, wherein there are sixteen magnetic separation boxes arranged in equi-spaced relation around the annular area of said discoidal rotor, and wherin two additional zones of strong magnetic field are provided to form additional air-gaps at 90* relationship to said first pair of air-gaps, said four air-gaps each being of such a width that two magnetic separation boxes can be accommodated therein simultaneously whilst a pair of separation boxes on each side thereof is in a zone of zero magnetic field.
 5. A magnetic separator for separating solid magnetic particles from a fluid current including a pair of magnetic separation boxes; a series of flat plates located in each magnetic separation box, said plates having triangular grooves on the lower major surface thereof, the apices of said grooves contacting the upper major surface of the next lower plate; upper and lower U-shaped magnetic pieces; means for supporting the upper magnetic piece in relation to the lower magnetic piece to form a pair of air-gaps therebetween in which the magnetic separation boxes are located in inclined relation to the horizontal; energizing coils carried at end portions of the U-shaped members for providing a magnetic field across the air-gap when said coils are energized thereby to sttrongly magnetize the plates of the separation box for removing magnetic particles from a fluid current, during a fluid current feed cycle, said removed magnetic particles adhering to said plates; a main supply of electric current for said coils for strong magnetization of said plates; means for passing a small electric current reverse to that of said main supply through said coils for reducing the remanent magnetic field to substantially zero when said main supply is interrupted during a scouring period to thereby remove the magnetic particles adhering to the plates of the separation box; and jacking means for raising the upper U-shaped magnetic piece in relation to the lower magnetic piece to facilitate removal of the separation boxes from said air-gaps.
 6. A magnetic separator according to claim 5, wherein said upper U-shaped magnetic piece is carried by a pair of rods, and wherein an arch-support carries a pair of lifting jacks to which the ends of respective rods are attached.
 7. A magnetic separator according to claim 6, wherein the ttop-plate of the separator box is formed as a piston member having a radially extending peripheral flange, the face of the upper U-shaped magnetic piece bearing on the piston member, a packing member being provided between the walls of the separator box and the piston member, and guide bars being provided to cooperate with the upper U-shaped magnetic piece to prevent it sliding on the inclined surface of the separator boxes.
 8. A magnetic separator according to claim 5, wherein each separator box includes two dividing plates located between the triangular grooved plates so as to divide said plates into three groups within said separator box, said dividing plates being longer than the triangular grooved plates and arranged to extend out of the inlet side of the separator box in an upwardly inclined direction owing to the inclination of the separator box, said dividing plates thus forming together with a chute extending from the base of the separator box, three supply troughs for the three groups of triangular grooved plates. 